Research On Metal Compounds Modified Cathode Related Materials For High Performance Lithium-Sulfur Batteries

The theoretical specific energy of lithium-sulfur battery is 2600 Wh/Kg, which is much higher than the current secondary batteries. It also has the advantages of environment-friendly, low price, which is expected to become the next generation of commercial secondary battery. However,sulfur and its final discharge products' poor conductivity, the discharge intermediate product polysulfide's shuttle effect and the volume expansion during the discharge process lead to the low utilization rate of the active substance and poor cycle performance, which limits the application of lithium-sulfur battery. Recently researchers have found that metal compounds have a good effect on improving the performances of cathode-related materials for lithium-sulfur batteries. We prepared and characterized the properties of four metal hydroxide-coated carbon-sulfur cathode composites. At the same time, the properties of the bacterial cellulose with MgO intermediate layer and the bacterial cellulose intermediate layer were prepared and characterized. The effects of the materials on the battery's cycle performance and rate performance were studied, The main conclusions are as follows:1. Study on metal hydroxides modified cathode materials.Nano-aluminum hydroxide, nano-cobalt hydroxide, nano-cerium hydroxide and nano-nickel hydroxide coatings were prepared on the surface of the prepared carbon-sulfur composite by hydrothermal method. The battery with sulfur loading of 2.5 mg/cm2 were tested on electrochemical performance.The results show that the performance of nano-aluminum hydroxides coated carbon-sulfur composite is best, its first discharge specific capacity is 1192 mAh/g. After 80 cycles its discharge capacity remained at 797 mAh/g, the capacity retention rate is 67%, which is very good cycle performance. Its rate performance were also better than carbon-sulfur composites and other metal-hydroxide-coated carbon-sulfur composites. In addition, the carbon-sulfur composites treated with nano-metal hydroxides have better performance compared to untreated carbon-sulfur composites.2. Study on the intermediate layer of bacterial cellulose/MgO. We produced magnesium hydroxide on the surface of bacterial cellulose with in-situ synthesis. Then we carbonized it to obtain bacterial cellulose/MgO intermediate layer material. Batteries were assembled with sulfur loading(>2.5mg/cm2). Then we tested there electrochemical performances. Batteries with bacterial cellulose/MgO intermediate layer show excellent cycle performance and rate performance. The battery with the bacterial cellulose/MgO intermediate layer has the best cycle performance compared to the battery without the intermediate layer and the battery with the bacterial cellulose intermediate layer. Its first discharge capacity is 1334 mAh/g, which is very high. After 100 cycles its capacity is still 922 mAh/g.The capacity retention rate is 69.1%. And its rate performance is also the best among them. Its discharge capacity is still 304 mAh/g at 1.5C. So the bacterial cellulose/MgO intermediate layer is a good way to improve the performances of the lithium-sulfur battery.